This invention relates to drills and more particularly to drill controls for automatically changing the speed of a bit in response to the thrust and force exerted on the bit when drilling in a workpiece.
Reference is made to U.S. Pat. No. 4,648,756 which is commonly assigned. As indicated in that patent, in many instances a workpiece to be drilled is made up of a number of layers of different materials. These layers may consist of difficult to drill materials laminated with easier to drill materials. As an example, a typical workpiece might consist of one composed of greater than an inch of titanium that requires a drill speed of 360 RPM followed by an inch of aluminum which should be drilled at 4,000 RPM and then another 1/4 inch of stainless of steel which also must be drilled at a lower speed. For maximum drill efficiency there is a need for a means to automatically control the drill rates through the various materials at the preferred drilling rates.
One means of controlling the drill rate is to sense the thrust of the drill as it drills through the various laminated layers. For example, in U.S. Pat. No. 4,478,538 there is disclosed a torque limiter 3 which includes a retainer 39 (see FIG. 3) and three balls 40 (see FIG. 2). When the drill is subjected to a high torque condition the balls 40 are pressed against the disc 33 with a great force and a loud sound is made. In U.S. Pat. No. 4,413,936 a longitudinal pin 28 acts as a transmitter to transmit forces from the tool receiving sleeve 16 to a controller 31 to thereby control rotational speed of the tool at different stages of drilling. In U.S. Pat. No. 4,487,270 a torque control is disclosed employing a conductive plastic disc 13 which functions as a pressure dependent resistor in conjunction with a pressure plate 12 and an electrode plate 14. In U.S. Pat. No. 4,198,180 a thrust controlled drilling apparatus is disclosed wherein a thrust sensor 23 with an air valve means acts in conjunction with a servo-valve unit 12 to affect a governor 11.
There is a need for a particular thrust and torque sensor which can be easily adapted for use in conjunction with a drill yet is in itself sensitive enough to be employed with a computer for controlling drill speed when drilling through laminated workpieces.